Horseshoe nails are specifically designed to bend as they are driven into a hoof. Typically a farrier drives the horseshoe nail into the bottom of the hoof through a slot in the shoe and as the nail is driven into the hoof, it bends so that the tip is caused to pass out the side of the hoof. Once in this position, the farrier removes the tip of the nail, either by cutting or ringing it off, and drives the remaining shank downwards against the hoof. This secures the nail, and therefore the shoe, in place.
A typical horseshoe nail includes a shank of generally rectangular cross section, and a head located at one end of the shank. The shank has a front and a rear surface interconnected by opposing edge surfaces, the edge surfaces being flat and nearly parallel but slightly tapered along a major part of the shank. In the end quarter of the shank remote from the head the edge surfaces converge together to form the tip. The rear surface of the shank is substantially flat, whereas the front surface of the shank is also flat along the major part of the shank except adjacent the tip where the front surface is angled to converge towards the rear surface at the tip. This angled portion is designed to cause the nail to bend as it is driven into the hoof.
It is desirable that the nail exit the hoof at a particular distance from the shoe. A well shot horse will have a ring of horseshoe nails spaced around the hoof all exiting the hoof at approximately the same distance from the bottom of the hoof. If a nail bends too sharply, it can fracture off an edge of the hoof. If a nail does not bend sufficiently or bends the wrong way, it cannot be secured in place and, moreover, can lame the horse.
In analysing the way a horseshoe nail bends when it is being driven into a hoof, it is considered that whilst the loading to cause bending is initially induced on the angled portion or reaction region adjacent the tip, as the nail beings to bend this loading is transferred along the shank towards the head. As a result, the part of the shank which extends from the angled portion towards the head is the main part of the nail which bends and throughout the specification, reference to the "bending region" of the nail refers to this part of the shank.
Horseshoe nails have generally been formed from a slug of metal cut from wire or rod of a gauge similar to the intended mean diameter of the nail head. This slug is drawn or rolled out, typically by air hammers, to form the head and to reduce its cross-section while forming the characteristic rectangular shank profile. To reduce work hardening, the nail undergoes one or two annealing treatments during or following shaping. Once fully shaped, the shank is then sheared to form the tip.
A problem with previous horseshoe nails is that they do not bend in a consistent manner. A broad reason is a lack of uniformity in the structure and bending properties of the nails, despite a seemingly uniform method of formation. One common cause of non-uniformity is the presence of flaws formed in the nails during their manufacture. These flaws are a consequence of the complexity in the nail forming operation and are often present as surface defects, such as dimples, or inconsistencies within the microstructure of the nail. As a result of the flaws, the nails will often have a weakened area which may cause the nail to bend the wrong way, or, in the case of flaws occurring in the microstructure, the nails may have an area of increased hardness in the nail shank which may cause the nail to drive straight or not bend sufficiently. Surface defects can have marked effects on the bending performance of the nail particularly when they are located in the tip or reaction region, whereas flaws in the microstructure cause particular problems when they occur in the bending region of the shank.
While a farrier is able to detect surface defects on a visual inspection of the nail, it is not practical for the farrier to examine the nail to determine if there are flaws in the microstructure. Typically the only way the farrier detects these flaws in the microstructure is by the feel of the nail, in particular as the nail is being driven into the hoof and this usually can only be done by experience farriers.
Another known cause of inconsistent bending is the presence of a region of increased resistance within the hoof which causes the nail to be deflected from its optimal path.